The invention relates to a helicopter-borne synthetic aperture radar system having rotating antennae for transmitting and receiving radar pulses (ROSAR). The antennae are arranged at the ends of arms that rotate with the rotor, and are connected with the radar system, which has at least one transmitting module, one electronic module with a central control device, an image processor and a display.
A radar system having a synthetic aperture (abbreviated SAR: Synthetic Aperture Radar) based on rotating antennae, abbreviated ROSAR (ROSAR:Rotor-SAR), is suitable for a helicopter as the supporting platform. The basic characteristics with respect to ROSAR were described in German Patent Document DE 39 22 086 C1. The rotating movement of the rotor blades or the rotating movement of a turnstile arranged above the rotor head and connected with the rotor mast is utilized for producing the synthetic aperture. As a result, the ROSAR principle differs basically from a radar (SAR) with a linear antenna movement, as customary in the case of airplanes.
A ROSAR system covers the entire angular range of a full circle of 360 degrees. In this case, transmitting and receiving antennae of a rotating arm are always used for transmitting and receiving, the transmitting pulses for scanning being radiated during the rotating movement of the arm at a defined pulse repetition frequency, by way of the transmitting antenna, and the echo signals received by way of the receiving antenna. This also relates to the antennae of the other rotating arms. The rotating arm may be a rotor blade which, on the rotor head, is connected with the rotating rotor mast of a helicopter. On the other hand, it may also be constructed as an arm of a turnstile (also called antenna cross) which is positioned above the rotor mast and is connected with the latter. The turnstile rotates at the rotational speed of the rotor mast. The radar system is a radar which ensures a coherent signal processing and is housed onboard the helicopter.
ROSAR is used both for warning of obstacles and for imaging the ground. For this purpose, the antennae are arranged to xe2x80x9clookxe2x80x9d into the radial direction and are oriented above and below the horizon. The viewing direction of the radar system is determined by the viewing direction of the antennae which depends in turn on the rotating position of the rotor mast. (The viewing direction is that direction on a full circle in which transmitting pulses and echo signals are represented on the display in an image.)
German Patent Document DE 43 28 573 C2 discloses a further development of the ROSAR to an all-weather viewing system for helicopters. The complex image produced by means of the all-weather viewing system is displayed on a pertaining cockpit display.
In the closest state of the art (its further development for the all-weather viewing system), no connection is described between the viewing direction of the antenna of the radar system and the inertial axes of a helicopter as the supporting platform.
The circular-arc-shaped synthetic aperture is defined as the path of the phase center of the antenna, which path is covered during the sweep over the azimuth opening angle for a certain target on the ground. Since the antenna phase center is not designed for a linear movement in the flight direction, but rather for a rotational movement, the viewing direction of the radar is arbitrary with respect to the inertial axes of the helicopter. Inertial axes may essentially be the spatial axes of coordinates (X-axis, Y-axis, Z-axis). The viewing direction of the radar is therefore situated at an undefined position of a full circle.
Since the rotational speed of the rotor mast may also vary, it is also disadvantageous that the number of the radar pulses per rotation does not correspond precisely to the actual rotational speed of the rotor mast, which also adversely affects the imaging.
It is an object of the invention to provide a ROSAR system in which imaging on the display of the radar system can be aligned with one of the inertial axes of the helicopter.
Another object of the invention is to provide such a system in which the adverse influence on the imaging of a change in rotational rotor speed is further limited.
These and other objects and advantages are achieved by the ROSAR according to the invention, in which a signal generator arranged at the rotor of the helicopter generates signals indicative of rotating positions of the rotor. The latter signals are transmitted to an electronic module of the radar system, which marks the flank of an individual signal from the signal sequence for a rotation of the rotor. This flank of a signal is defined as a xe2x80x9cbasic positionxe2x80x9d and is counted. (The basic position is a point on the circular rotation of the rotor which marks the start of a rotor rotation; and the location of the basic position on the full rotational circle also simultaneously defines the position of an inertial axis of the helicopter. The location of the basic position on a full circle can be changed.) Furthermore, the central control unit uses the signal with respect to the basic position to generate a control signal that is used in the transmitter module to synchronize the pulse rate of that module with the rate of rotation of the rotor.
The signal generator that is used for this purpose comprises one or more transmitters arranged at the rotor, and one or more opposite receivers are arranged on the fuselage of the helicopter. (However, the transmitters and the receivers can also be positioned in a reverse sequence.) The transmitters may be magnets and the receivers may be corresponding Hall elements. Alternatively, the transmitters and receivers may be opto-electronic or microwave components.
The invention permits azimuthally phase-accurate synchronization between the viewing angle of a helicopter-borne radar system and an inertial axis of the helicopter, as well as a defined alignment of the image on the video screen with respect to an inertial axis of the helicopter (and thus with respect to the flight direction of the helicopter.) The relationship between the image content and the flight direction is provided solely by means of such synchronization. The viewing direction of the radar system (ROSAR) can be adjusted by means of an electric regulator wheel, so that the pilot can set a desired defined viewing direction of the radar image on the video screen.
The provisions of a signal generator at the rotor can also improve the imaging, in that the radar pulses are synchronized with respect to the rotational speed of the rotor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.